1. Field of the Invention
The present invention relates to a duplexer, and a laminate-type high-frequency device and communication equipment provided with the duplexer. More specifically, the present invention relates to an antenna duplexer sharing one antenna on a transmitting side and a receiving side and separating a transmitted signal and a received signal from each other, and a laminate-type high-frequency device and communication equipment provided with the antenna duplexer.
2. Description of the Related Art
In an antenna duplexer sharing one antenna on a transmitting side and a receiving side, and separating a transmitted signal and a received signal from each other, two filters having different center frequencies are connected. Therefore, it is required that, at a center frequency of one of the two filters, the impedance of the other filter seen from an antenna that is an input/output terminal shared by the two filters is set to be infinite, whereby the transmission characteristics of the one of the two filters are prevented from being impaired.
For this purpose, a duplexer as shown in FIG. 24 is proposed (e.g., see pages 4–5 and FIG. 3 in JP 3,204,753). FIG. 24 is an exploded perspective view of the duplexer in the prior art.
As shown in FIG. 24, a first end ground electrode (not shown) is provided on a lower surface of a dielectric layer 11p. Furthermore, a through-hole 91p for connecting the first end ground electrode to a tip portion of a transmission line 70p (described later) is formed in the dielectric layer 11p. 
On an upper surface of a dielectric layer 14p, resonant elements 21p, 22p, 23p, 24p, and 25p are provided, each having one end that is connected to the first end ground electrode, respectively, to form a ¼ wavelength stripline resonator. Furthermore, on the upper surface of the dielectric layer 14p, electrodes 31p, 32p, 33p, 34p, and 35p are provided, each having one end that is connected to the first end ground electrode, respectively, and the other end that is opposed to open ends of the resonant elements 21p, 22p, 23p, 24p, and 25p, respectively, with a predetermined gap. A filter 500p in a comb-line shape is configured by using the distribution coupling of the resonant elements 21p and 22p, and a filter 600p in a comb-line shape is configured by using the distribution coupling of the resonant elements 23p, 24p, and 25p. Furthermore, on the upper surface of the dielectric layer 14p, the transmission line 70p for constituting an inductor between the resonant elements 22p and 23p is provided. A through-hole 94p is formed at the tip portion of the transmission line 70 so as to be connected to the first end ground electrode.
On an upper surface of a dielectric layer 12p, an inner layer ground electrode 81p positioned on an open end side of the resonant elements 21p and 22p and connected to the first ground electrode is provided, and dielectric layers 13p and 14p are interposed between the inner ground electrode 81p and the open end side of the resonant elements 21p and 22p. Furthermore, on the upper surface of the dielectric layer 12p, an inner layer ground electrode 83p positioned on an open end side of the resonant elements 23p, 24p, and 25p and connected to the first end ground electrode is provided, and the dielectric layers 13p and 14p are interposed between the inner layer ground electrode 83p and the open end side of the resonant elements 23p, 24p, and 25p. Furthermore, a through-hole 92p for connecting the first end ground electrode to the tip portion of the transmission line 70p is formed in the dielectric layer 12p. 
On an upper surface of the dielectric layer 13p, a capacitive coupling electrode 50p is provided, one end 51p of which is overlapped with a part of the transmission line 70p with the dielectric layer 14p interposed therebetween and the other end 52p of which is overlapped with a part of the resonant element 23p with the dielectric layer 14p interposed therebetween. Furthermore, on the upper surface of the dielectric layer 13p, an input/output electrode 42p overlapped with a part of the resonant element 25p with the dielectric layer 14p interposed therebetween is provided. A through-hole 93p for connecting the first end ground electrode to the tip portion of the transmission line 70p is formed in the dielectric layer 13p. 
The tip portion of the transmission line 70p is short-circuited with the first end ground electrode provided on the lower surface of the dielectric layer 11p via the through-holes 94p, 93p, 92p, and 91p. The through-hole 94p is positioned so that an electric length of the transmission line 70p is a predetermined length of 90 degrees or less. Thus, the transmission line 70p constitutes an inductor.
On an upper surface of the dielectric layer 15p, a capacitive electrode 60p is provided, one end 61p of which is overlapped with a part of the transmission line 70p with the dielectric layer 15p interposed therebetween, and the other end 62p of which is overlapped with a part of the resonant element 22p with the dielectric layer 15p interposed therebetween. Furthermore, on the upper surface of the dielectric layer 15p, an input/output electrode 41p is provided, a part of which is overlapped with a part of the resonant element 21p with the dielectric layer 15p interposed therebetween.
A second end ground electrode 110p is provided on an upper surface of a dielectric layer 17p. 
On an upper surface of a dielectric layer 16p, an inner layer ground electrode 82p positioned on the open end side of the resonant elements 21p and 22p and connected to the second end ground electrode 110p is provided, and the dielectric layers 15p and 16p are interposed between the inner layer ground electrode 82p and the open end side of the resonant elements 21p and 22p. Furthermore, on the upper surface of the dielectric layer 16p, an inner layer ground electrode 84p positioned on the open end side of the resonant elements 23p, 24p, and 25p and connected to the second end ground electrode 110p is provided, and the dielectric layers 15p and 16p are interposed between the inner layer ground electrode 84p and the open end side of the resonant elements 23p, 24p, and 25p. 
The dielectric layers 11p, 12p, 13p, 14p, 15p, 16p, a above-mentioned configuration are laminated to be integrated, followed by sintering, providing a duplexer composed of a laminate, in which dielectric layers and electrode layers are laminated alternately.
FIG. 25 shows an equivalent circuit of the duplexer having the above-mentioned configuration. As shown in FIGS. 24 and 25, a band-pass filter 500p is composed of the resonant elements 21p and 22p, and a band-pass filter 600p is composed of the resonant elements 23p, 24p, and 25p. A static capacitor 401p is formed between the capacitive electrode 60p and the transmission line 70p, and a static capacitor 402p is formed between the capacitive electrode 50p and the transmission line 70p. The inductor 403p is composed of the transmission line 70p. The inductor 403p is connected in parallel with the filters 500p and 600p, and the static capacitor 401p is connected in series between an antenna 700p and the filter 500p. The static capacitor 402p is connected in series between the antenna 700p and the filter 600p. The inductor 403p, and the static capacitors 401p and 402p constitute a branching circuit 400p. 
FIG. 26 shows the transmission characteristics of a duplexer having the above-mentioned configuration. As shown in FIG. 26, it is understood that the branching circuit 400p shares one antennal on a transmitting side and a receiving side, and separates a transmitted signal and a received signal from each other.
As described above, the duplexer in the prior art includes a branching circuit provided between two filters in a triplet shape, in which resonant elements are provided integrally in a dielectric substrate. The branching circuit is configured by using static capacitors connected in series to each of the two filters and an inductor connected in parallel therewith.
However, in the above-mentioned configuration, the circuit configuration is complicated, so that a loss in the branching circuit is increased, and the layout space is necessarily increased. Furthermore, it is difficult to obtain a large L-value in the inductor connected in parallel with two filters, respectively, so that the degree of design freedom is lowered.